There should be no doubt among anyone who has seen any of Errol Morris’ films (which began with 1978 profound “Gates of Heaven”) that he is among our greatest living documentarians. Few have so incisively probed their subjects’ knotty moral grounds while also pushing the form of the medium. While the talented filmmaker’s latest “The Unknown Known” can be called another triumph, this is a film as much about what Morris is able to extract from his subject as what can only be read between the lines.

That subject is Donald Rumsfeld, a Secretary of Defense under President Gerald Ford and, more infamously, under George W. Bush, from 2001 to 2006. The title refers to Rumsfeld’s answer to a press conference question in February 2002, regarding the mounting decision to invade Iraq. It’s a cruel twist of fate that, as the film is now available for all to see, the destabilizing effects of American presence, commenced over a decade ago under this man’s tutelage, have once again rearing their head in the form of ISIS.

Rumsfeld’s twisted logic and cryptic doublespeak is largely baffling even to Morris, who would go on to refer to it as “gobbledygook” in press for the film.

In 2004, Morris made a similar film, entitled “The Fog of War,” in which an extremely candid and humble Robert S. McNamara spoke of his experiences, and mistakes, as Secretary of Defense during Vietnam. Clearly, this was a man who considered the ramifications of warfare and how such could or could not be justified. Rumsfeld, by comparison, is a clam, a man apparently refusing to admit – even to himself – of his gross misuse of the public trust afforded to him. “The Unknown Known” is an excellent companion piece to the former film, and should also be mandatory viewing for those who want to better understand recent world history. It is, by definition, nonfiction, but at times suggests a horror movie.